The small
alpine glaciers that once existed 120 kilometers east of Los Angeles, Calif.,
were, according to a new study, present as recently as 5,000 years ago, a time
when regions to the north had already become ice-free. The research may thus upend
some thinking about the paleoclimate of California.

To determine when the ice receded, researchers at the University of California,
Riverside (UCR), and Lawrence Livermore National Laboratory used cosmogenic radionuclide
dating, also called surface exposure dating, to date boulders from moraines on
San Gorgonio Mountain in the San Bernardino range. The moraines mark the extreme
southwestern limit of glaciation in the United States and represent several episodes
of late Quaternary glaciation.

Lewis Owen, pictured here in the Sierra
Nevada at the southern end of the western cordillera, and colleagues have dated
boulders from nearby moraines at ages younger than expected  perhaps reshaping
understanding of Californias paleoclimate. Photo courtesy of University
of California, Riverside.

Geomorphologist Lewis Owen and colleagues report in the August Geology that
the moraines ages clustered to indicate four stages of glacial advance:
20,000 to 18,000 years ago, 16,000 to 15,000 years ago, 13,000 to 12,000 years
ago and 9,000 to 5,000 years ago.

Earlier studies based on the relative amount of weathering and degree of soil
formation had found the oldest moraines to be up to 80,000 years old. And although
some of the new ages for the moraines are consistent with other more recent studies,
the youngest stage is not consistent with the glacial record just 250 kilometers
to the north, says Alan Gillespie, a Quaternary geologist at the University of
Washington.

The Sierra Nevada is thought to have been ice-free during the mid-Holocene,
at the same time Owen concludes glaciers were advancing on San Gorgonio,
Gillespie says. If the claimed mid-Holocene advance at San Gorgonio can
be confirmed, it does imply that the paleoclimate in California, and elsewhere,
may be more spatially variable than we have previously been able to model or document.

The authors acknowledge that the exposure ages of boulders in the youngest moraine
are not as tightly grouped as the older moraines. However, they say that the dates
are nonetheless consistent with a Holocene cold snap called the Neoglacial that
occurred 4,000 to 5,000 years ago, as well as a cooling around 8,500 years ago
brought on by the collapse of the Laurentide ice sheet.

Surface exposure dating, which uses ratios of beryllium-10, has recently become
a popular means of establishing ages for glacial landforms. In rocks exposed at
Earths surface, cosmic rays interact with the silicon and oxygen in quartz
to produce beryllium-10. The more beryllium-10 that has accumulated, the longer
the rock has been exposed at the surface.

However, one has to be aware of the limitations, says co-author Robert
Finkel of Lawrence Livermore. With surface exposure dating, the younger the sample
is, the higher the degree of uncertainty because there are fewer nuclides available
to measure. This limitation may also help explain the wide age scatter of boulders
in the youngest moraine.

Owens team also notes that the discrepancy with the Sierra Nevada shows
that a local characteristic of San Gorgonio Mountain  blowing snow 
can be an important, though often overlooked, factor in glacier formation.
The lateral transport of snow and high winds during storms resulted in much
heavier deposition of snow into these cirques, says co-author Richard Minnich,
an ecologist at UCR, and that was the support of these glaciers.

The timing of glacial advance is important because small mountain glaciers are
particularly sensitive to climate change and, with accurate dating, can be used
as proxies in climate models. In Southern California, well-dated moraines could
help paleoclimatologists reconstruct the past locations of the jet stream.

The study has another implication, for modern times. Owen and his colleagues say
the recent glacial advance shows that under certain conditions, snowpack can become
perennial even at relatively low latitudes and low altitudes  meaning, they
say, that small glaciers could exist there again with just a slight change in
climate.

We can already demonstrate that permanent perennial snow still occurs on
this mountain in about one-third of the years, Minnich says. So a
slight increase in the annual precipitation here in the West could, in fact, produce
permanent snow and ice  enough to make moraines, even in the modern Holocene
climate.